1. Field of the Invention
This invention relates to a method of manufacturing an electron-emitting device having an electroconductive film, an electron source realized by arranging a plurality of such electron-emitting devices on a substrate and an image-forming apparatus comprising the same.
2. Related Background Art
CRTs have been widely used for image-forming apparatus for displaying images by means of electron beams.
In recent years, on the other hand, flat panel display apparatus utilizing liquid crystal have been replacing CRTs to some extent. However, they are accompanied by certain drawbacks including that they have to be provided with a back light because they are not not emissive and hence there exists a strong demand for emissive type display apparatus. While plasma displays have become commercially available as emissive type display apparatus, they are based on principles that are different from those of CRTs and cannot fully compete with CRTs, at least currently, from the viewpoint of contrast, chromatic effects and other technological factors. Since an electron-emitting device appears to be very promising for preparing an electron source by arranging a plurality of such devices and an image-forming apparatus comprising such an electron source is expected to be as effective as CRT for light emitting effects, efforts have been made in the field of research and development of such electron-emitting devices.
For instance, the applicant of the present invention has made a number of proposals for an electron source realized by arranging a number of surface conduction electron-emitting devices that are cold-cathode type devices and an image-forming apparatus comprising such an electron source.
Since the configuration and the characteristic features of a surface conduction electron-emitting device and those of an electron source comprising such devices are described in detail in various documents, including Japanese Application Laid-Open No. 7-235255, they will be described only summarily here. FIGS. 4A and 4B of the accompanying drawings schematically illustrate a surface conduction electron-emitting device comprising a substrate 1, a pair of device electrodes 2 and 3 and an electroconductive film 4, which includes an electron-emitting region 5. With a method of producing an electron-emitting region, a part of the electroconductive film is deformed, transformed or destroyed to make it electrically highly resistive by applying a voltage between the paired device electrodes. This process is referred to as "energization forming process". In order to produce an electron-emitting region that operates well for electron emission in an electroconductive film, the latter preferably comprises electroconductive fine particles such as fine particles of palladium oxide (PdO). A pulse voltage is preferably used for an energization forming process. A pulse voltage to be used for energization forming may have a constant wave height as shown in FIG. 13A or, alternatively, it may have a gradually increasing wave height as shown in FIG. 13B.
While an electroconductive film of fine particles may be prepared by means of a gas deposition technique, with which electroconductive fine particles are deposited directly on a substrate, a technique of applying a solution of a compound of the element that constitutes the electroconductive film (e.g., an organic metal compound) to a substrate and producing a desired electroconductive film, typically by heat treatment, is more advantageous particularly for preparing a large electron source because it does not require the use of a vacuum apparatus and hence is less costly. For applying a solution of an organic metal compound only to an intended area, an ink-jet device may advantageously be used because it does not require any additional patterning operation for the electroconductive film.
After producing an electron-emitting region, a film containing carbon as a principal ingredient is formed by deposition in the electron-emitting region and its vicinity, to increase the intensity of electric current flowing through the device and improve the electron-emitting property of the device a pulse voltage is applied between the device electrodes in an appropriate atmosphere containing organic substances (a process referred to as "activation process").
Then, the electron-emitting device is preferably subjected to a process referred to as "stabilization process", where the device is placed into and heated in a vacuum vessel, while the latter is gradually evacuated, in order to satisfactorily remove the organic substances remaining in the vacuum vessel and make the device operate stably.
Methods for producing electroconductive films for an electron source comprising surface conduction electron-emitting devices are disclosed in a number of documents including Japanese Patent Application Laid-Open No. 8-273529, the assignee of which is the applicant of the present patent application.
Now, ink-jet devices that can be used for the purpose of the present invention will be briefly described below.
Ink-jet devices are roughly classified into two types according to the ink ejection technique used in the device.
According to a first ink ejection technique, fine liquid drops of ink are ejected by the pressure generated by contraction of a piezo-electric element arranged in a nozzle. A second technique is referred to as a bubble-jet system, with which ink is heated to a bubble by means of a heat-generating resistor and then ejected in the form of fine liquid drops.
FIGS. 5 and 6 schematically illustrate ink-jet devices of these two types.
FIG. 5 shows a piezo-jet type ink-jet device comprising a first glass-made nozzle 21, a second glass-made nozzle 22, a cylindrical piezo-electric element 23, tubes 25 and 26 for feeding liquid to be ejected, that may typically be a solution of an organic metal compound, and an electric signal input terminal 27. As a predetermined voltage is applied to the electric signal input terminal, the cylindrical piezo-electric element contracts to discharge the liquid staying there as fine drops.
FIG. 6 shows a bubble-jet type ink-jet device comprising a base plate 31, a heat-generating resistor 32, a support plate 33, a liquid path 34, a first nozzle 35, a second nozzle 36, a partition wall 37, a pair of liquid chambers 38 and 39 containing a predetermined liquid, a pair of liquid supply ports 310 and 311 and a top plate 312. With this arrangement, the liquid in the liquid chambers is caused to bubble and forced out from the nozzles as liquid drops by the heat generated by the heat-generating resistor. While each of the above-described devices has a pair of nozzles, the number of nozzles arranged in a device of the type under consideration is not limited to two.
After applying a solution of an organic metal compound only to predetermined areas as fine liquid drops by means of an ink-jet device of either of the above-described types and then drying the solution, the organic metal compound is heated for pyrolysis to produce an electroconductive film typically made of fine particles of metal or metal oxide.
The resulting electroconductive film has a thickness preferably between several and 50 nanometers, although it may vary depending on the electric resistance of the electroconductive film, the distance separating the device electrodes and other factors. The variance of the film thickness has to be strictly limited within a single electron-emitting device and also among the electron-emitting devices of an electron source.
An electron-emitting region may not be prepared correctly and properly in an electron-emitting device if the electroconductive film of the electron-emitting device shows a large variance. Likewise, an electron source comprising a large number of electron-emitting devices showing a large variance in the film thickness of their electroconductive films may not operate evenly and uniformly for electron emission.
Therefore, the ink-jet device to be used for producing electroconductive films has to be examined and regulated thoroughly in order to ensure an even and uniform production of electroconductive films that are free from any undesirable variance in the film thickness.
A large and high definition flat-type image-forming apparatus can be manufactured only by using an electron source comprising a large number of electron-emitting devices that operate satisfactorily from the above described point of view.
Thus, while the ink-jet device being used for forming electroconductive films on respective electron-emitting devices is rigorously controlled for operation in order to avoid producing defective devices, the probability of producing defective devices inevitably rises as the number of electron-emitting devices arranged in an image-forming apparatus increases.
There can be various causes that give rise to defective electroconductive films produced by means of an ink-jet device, including noise mingled into the electric signals for controlling the ink-jet device that interfere with the normal liquid drop ejecting operation of the device to make the film thickness of the produced electroconductive film significantly depart from a predetermined level, mechanical vibrations that displace the locations where liquid drops are applied on the electron source substrate, and foreign objects put into the liquid contained in the ink-jet device to interfere with the normal liquid discharge of the device to make the electroconductive films unacceptable in terms of thickness, location and profile.
When manufacturing electron-emitting devices on a mass production basis, it is very difficult to improve the rate of producing acceptable devices or the manufacturing yield particularly when a large number of electron-emitting devices have to be produced on a single substrate.
A high manufacturing yield is accompanied by high manufacturing cost and a need for treating rejected devices. In view of the current social need for suppressing the volume of industrial waste, therefore, there is a strong and urgent demand for a method of manufacturing electron-emitting devices at a high yield.